Dragline excavators may be categorised as falling within one of two general types; so-called “Conventional” draglines and Universal Dig Dump (“UDD”) draglines. A Conventional dragline employs a single hoist rope and, to enable angular adjustment of the dragline bucket, a dump rope connects the drag rope to the forward end of the bucket by way of a pulley that, in turn, is connected with the hoist rope. By way of contrast, a UDD dragline employs two, forward and rearward, hoist ropes that are rigged to be operated independently of one another and, thus, without any interconnecting dump rope.
In the case of the Conventional dragline, the single hoist rope is connected to opposite side walls of the bucket by way of splayed hoist chains. Similarly, in the case of the UDD dragline, the rearward hoist rope is connected to opposite side walls of the bucket by way of splayed hoist chains. In each case the hoist chains are connected to lower, rearward regions of the respective side walls of the bucket by way of axially aligned trunnions. Also, in each case and in order to provide for clearance between the spayed hoist chains and the side walls of the bucket during tilting/turning of the bucket about the axis of the trunnions, a spreader bar is employed to increase the size of the effective splay angle between the hoist chains, and the effective width of the bucket is reduced (i.e., the side walls are tapered inwardly) in the region in which interference might occur between the hoist chains and the side walls of the bucket.
Depending upon the type of dragline and the bucket size employed in any given case, the spreader bar typically has a weight within the range 500 kg to 2,000 kg, and this creates two problems. It imposes a commensurate weight reduction on the bucket payload during each operating cycle, and bucket damage is regularly experienced due to collisions occurring between the spreader bar and the bucket during excavating operations. Also, with the bucket width being reduced to avoid interference with the hoist chains, the payload volume is reduced commensurately during each operating cycle.
Three different approaches are known to have been taken toward obviating the need for the spreader bar and thereby minimising the above mentioned problems. In one case the hoist chain trunnions have been moved toward the upper rim of the side walls of the bucket, but this has created problems with load dumping as a consequence of the tilting axis of the bucket being shifted to a level above that of the centre of gravity of the bucket. In another case, the hoist chain trunnions have been moved into the interior of the bucket, but this has resulted in a reduction in the payload capacity of the bucket and interference with the hoist chains. In the third case the hoist chain trunnions have been moved to the rear wall of the bucket but positioned at or below the level of the centre of gravity of the bucket and angled to correspond approximately with the splay angle of hoist chains. This results in excessive wear due to the trunnion axes being inclined with respect to the tilting axis of the bucket.